210407_문자열(String)

문자열(String)

문자 하나는 character이며, char이라는 축약어로 부르기도 합니다. char가 포함된 메소드도 있습니다.
MDN 문자열
문자열 블로그

as primitives, from string literals / as objects, using the String() constructor

Creating strings

using '' / "" / `${any}`

String operators

string + 다른타입 = string
word1 + " " + word2
let iwantTo = 'behappy';
iwantTo += 'behappy'; // evaluates to 'iwantTobehappy'

length of a string

• str.length;

let iwantTo = 'behappy';
iwantTo.length; //return 7

Retrieving a specific string character

• str[index]
  -include square brackets ([]) on the end of your variable name
  -문자열의 글자 하나하나에 접근가능
  -str.charAt() method : other way

iwantTo[0]; ->first letter of..
iwantTo[iwantTo.length-1];->last character of..

iwantTo[0] = 'c'; -> doesn't work( read-only)

Finding a substring inside a string and extracting it

• str.indexOf(searchValue [, fromIndex])
  -('찾을 값'[, 시작인덱스])
  -indexOf라는 method(함수)-
• str.lastIndexOf(searchValue [, fromIndex])
  -the last occurrence of searchValue
• str.includes(searchString[, position])
  -one string may be found within another string, returning true or false

<to find, fillter string ..etc>
iwantTo.indexOf('happy'); // 2 ->starts at position 2 

<find all instances of strings that don't(or do) contain the substring 'behappy'>
if(iwantTo.indexOf('behappy') === -1) {
  // do stuff with the string if the 'behappy'
  // substring is NOT contained within it
}

if(iwantTo.indexOf(behappy') !== -1) {
  // do stuff with the string if the 'behappy'
  // substring IS contained within it
}

'undefined'.indexOf() // return 0, as undefined is found at position 0 in the string undefined. 
'undefine'.indexOf() // return -1, as undefined is not found in the string undefine.

For fromIndex values lower than 0, or greater than str.length, the search starts at index 0 and str.length
'beHappy'.indexOf('H',-2)  // 2
'beHappy'.indexOf('H', 10) // -1


With no fromIndex value, or any fromIndex value <= string's length : return value is the same as the fromIndex value

'hello world'.indexOf('') // returns 0
'hello world'.indexOf('', 0) // returns 0
'hello world'.indexOf('', 3) // returns 3

any fromIndex value >= the string's length : return the string's length:

'hello world'.indexOf('', 11) // returns 11
'hello world'.indexOf('', 13) // returns 11

'canal'.lastIndexOf('');      // returns 5
'canal'.lastIndexOf('', 2);   // returns 2
'cafcaf'.lastIndexOf('caf',0) // 0
'cafcaf'.lastIndexOf('caf',-10) // 0
'cafcaf'.lastIndexOf('caf',10) // 3

const str = 'To be, or not to be, that is the question.'

console.log(str.includes('To be'))  // true
console.log(str.includes('nonexistent'))  // false
console.log(str.includes('To be', 1))     // false
console.log(str.includes(''))             // true

• str.slice(beginIndex[, endIndex])
  - extracts a section of a string and returns it as a new string, without modifying the original string.

  -0, 양수일때 왼-오 방향/ 음수일때 오-왼 방향

let str1 = 'The morning is upon us.', // the length of str1 is 23.
    str2 = str1.slice(1, 8) // OUTPUT: he morn
    str3 = str1.slice(4, -2)// OUTPUT: morning is upon u
    str4 = str1.slice(12)// OUTPUT: is upon us.
    str5 = str1.slice(30) // OUTPUT: ""

let str = 'The morning is upon us.'
str.slice(-3)      // returns 'us.'
str.slice(0, -1)   // returns 'The morning is upon us'
str.slice(-11, 16) // => "is u"

• str.substring(indexStart[, indexEnd])
  value < 0 // as if it were 0
  value > stringName.length // stringName.length,

value === NaN // as if it were 0.(0번째 인덱스부터)

• str.substr(startIndex, from startIndex~문자개수)

The substring() : swaps its two arguments if indexStart > indexEnd
The slice() : returns an empty string if this is the case.

let text = 'Mozilla'
console.log(text.substring(5, 2))  // => "zil"
console.log(text.slice(5, 2))      // => ""

Changing case

• toLowerCase()
• toUpperCase()
  - if you want to normalize all user-entered data before storing it in a database.
  

let iwantTo = 'beHappy';
iwantTo.toLowerCase(); // "behappy"
iwantTo.toUpperCase(); //"BEHAPPY"

• str.replace(regexp|substr, newSubstr|function)

const iwantTo = iwantTo.replace('Happy','Smart')
(실제 변수값 바꾸려면 할당시켜야)->다른 method들도 마찬가지

• str.split([separator[, limit]])
  -divides a String into an ordered list of substrings, puts these substrings into an array, and returns the array. The division is done by searching for a pattern;

const str = 'The quick brown fox jumps over the lazy dog.';

const words = str.split(' ');
console.log(words[3]); // "fox"

const chars = str.split('');
console.log(chars[8]); // "k" ->empty 스트링일 경우 단어 하나하나 각각 array

const strCopy = str.split();
console.log(strCopy); //  ["The quick brown fox jumps over the lazy dog."] ->아예 빈칸일 경우 스트링 전체가 하나의 array

arr.join([separator])
-creates and returns a new string by concatenating all of the elements in an array (or an array-like object), separated by commas or a specified separator string.

var a = ['Wind', 'Water', 'Fire'];
a.join();      // 'Wind,Water,Fire'
a.join(', ');  // 'Wind, Water, Fire'
a.join(' + '); // 'Wind + Water + Fire'
a.join('');    // 'WindWaterFire

• Reversing a String using split()

const str = 'asdfghjkl'
const strReverse = str.split('').reverse().join('')
// 'lkjhgfdsa'

// split() returns an array on which reverse() and join() can be applied

String (): return 문자열 프리미티브
new String (): 객체 문자열

 var x= String('word');
    console.log(typeof x); //"string"
    var y= new String('word');
    console.log(typeof y); //"object"
    console.log(new String('')=== new String('')) //false!!!

str1.concat(str2, str3...);

-join two or more strings.
var  str1 = "Hello ";
var  str2 = "world!";
var  str3 = " Have a nice day!";
var  res = str1.concat (str2, str3); // -->Hello world! Have a nice day!

str.trim()
문자열을 양쪽 끝 공백을 제거

    var Sample = " Hello "
   console.log(Sample.trim())  //"Hello"

\t : tab
\r : carriage return 
\n : new line(줄 바꿈)
Template literals 안에서는 그냥 엔터 가능
str.match(regexp) : return an Array whose contents depend on the presence or absence of the global (g) flag, or null if no matches are found.
RegExp : regular expressions

<Using indexOf() to count occurrences of a letter in a string>

const str = 'To be, or not to be, that is the question.'
let count = 0
let position = str.indexOf('e')

while (position !== -1) {
count++
position = str.indexOf('e', position + 1)
}

console.log(count) // displays 4

console.log('string text line 1\n' +
'string text line 2');
// "string text line 1
// string text line 2"

console.log(string text line 1 string text line 2);
// "string text line 1
// string text line 2"

const five = 5;
const ten = 10;
console.log('Fifteen is ' + (five + ten) + ' and not ' + (2 * five + ten) + '.');
// "Fifteen is 15 and not 20."
Now, with template literals, you are able to make use of the syntactic sugar making substitutions like this more readable:

const five = 5;
const ten = 10;
console.log(Fifteen is ${five + ten} and not ${2 * five + ten}.);
// "Fifteen is 15 and not 20."